Not Applicable
This invention relates generally to a new method for applying a plurality of cutting edges to a tool for cutting or milling downhole metal items, such as fixed casing strings in a well bore, and to a new type of cutting element used in this method.
Heretofore, cutting tools for cutting metal items downhole, such as well casing or casing strings, have been provided with one of two types of cutting elements mounted on the cutting surfaces of the cutting tool. Generally, the two types of cutting elements which have been used are an aggregate of crushed tungsten carbide particles, or a pattern of whole cutting inserts. The cutting surfaces on which the aggregate or whole inserts are mounted have been fixed blades, swinging blades, or the tool body itself, depending upon the intended function of the tool.
Generally, the whole insert type of cutting element has been made of cutting grade tungsten carbide, in the shape of discs, triangles, rectangles, parallelograms, or other shapes. These inserts have been bonded to the cutting tool, sometimes in a uniform pattern, and sometimes in a random pattern. The random pattern is easier and more economical to apply, but the uniform pattern has been more effective. Most of these whole inserts have had generally flat front faces, and generally flat rear faces, with the rear face being bonded to the cutting tool, and the front face being presented to the workpiece as a cutting face. Most of these inserts also have substantially parallel front and rear faces. One other known insert is a pyramid shape having four flat triangular sides.
It is important to cut relatively short, thick, metal chips from the workpiece, to allow efficient removal of the chips from the well, via the flow of drilling fluid. If this is not done, long, thin, stringy metal chips can be formed. These long, thin chips can adhere together, in a xe2x80x9cbird nestingxe2x80x9d effect, which can clog the drilling fluid flow passages.
In order to promote the cutting of relatively short metal chips from the workpiece with the whole cutting inserts, at least two types of features sometimes have been employed. One such feature has been the provision of a surface irregularity on the front face of each cutting insert, to curl each metal chip back toward the workpiece until it breaks off at a relatively short length.
The second such chip breaking feature has been to tilt the front face of each insert at a non-orthogonal attack angle relative to the surface of the workpiece. In this context, the term xe2x80x9crake anglexe2x80x9d has been used to refer to the condition where one portion of the front face of the insert is advanced ahead of another portion, in the direction of rotation of the cutting tool. The degree of advancement is usually small, with approximately 20xc2x0 being the upper limit, resulting in an angle between the front face of the cutting insert and the surface of the workpiece of 70xc2x0 to 90xc2x0. The rake angle can be xe2x80x9cpositivexe2x80x9d or xe2x80x9cnegativexe2x80x9d, depending upon which portion of the front face of the cutting insert is advanced. If the leading portion of the front face contacts the workpiece surface, a xe2x80x9cpositivexe2x80x9d rake angle is said to exist. If the trailing portion of the front face contacts the workpiece surface, a xe2x80x9cnegativexe2x80x9d rake angle is said to exist. The use of a rake angle can cause the front face of the insert to xe2x80x9cdragxe2x80x9d across the workpiece, or to xe2x80x9cgougexe2x80x9d the workpiece, depending upon the particular type of rake angle employed, and depending upon the contour of the cutting portion of the insert. A negative rake angle is generally considered to achieve the best chip breaking effect.
The front face of the insert can have a xe2x80x9cradialxe2x80x9d rake angle, where the front face lies in a plane which is parallel to the rotational axis of the cutting tool, but which extends non-radially from the cutting tool. Or, the front face of the insert can have an xe2x80x9caxialxe2x80x9d rake angle, where the front face lies in a plane which intersects, but does not contain, the rotational axis of the cutting tool. Or, the front face of the insert can have a xe2x80x9ccompoundxe2x80x9d rake angle having both radial and axial components.
A rake angle on the front face of the cutting insert can be the result of an angle on the cutting tool surface on which the cutting insert is mounted, or an angle between the front face of the cutting insert and the rear face, or both.
When tungsten carbide aggregate has been used as the cutting elements instead of whole cutting inserts, it has not been possible to employ either of the two chip breaking features discussed above. The tungsten carbide particles used as cutting elements in the aggregate are not uniform either in material or in conformation. They are typically made by crushing whole inserts or worn out tungsten carbide machine components, such as extrusion dies, rollers, or hammers. This produces a wide assortment of shapes and sizes of particles, or chunks, of varying formulations of tungsten carbide material. Some of these particles or chunks are not even xe2x80x9ccutting gradexe2x80x9d tungsten carbide, and some of the faces or edge profiles of these particles or chunks are not suitable for use on cutting elements. Even where surface irregularities are present on the crushed carbide particles, they are not uniformly distributed or optimally arranged on the front face of each cutting element, so their effect is greatly reduced or eliminated.
The crushed aggregate is typically applied to the cutting tool in a more or less random pattern, and each particle is randomly oriented on the surface of the cutting tool. In one method, the crushed aggregate is formed into a solid bar by randomly suspending the particles in a matrix of brazing material, such as a nickel/brass matrix. The bar is then bonded to the cutting tool as a unit. In another method, the crushed aggregate is randomly suspended within a welding rod and then bonded directly to the cutting tool, by melting of the welding rod onto the cutting tool. In either method, it is impossible to control the orientation of each particle of tungsten carbide relative to the cutting tool. Therefore, it is impossible to control the angle at which the leading face or leading edge of each particle is ultimately presented to the workpiece. Further, it is difficult to arrange the particles in a uniform pattern on the cutting tool, since the particles are not of uniform size and shape. Even though the technician typically attempts to pack the crushed particles together for good coverage, some areas will have a higher concentration of smaller carbide particles, with few open spaces therebetween, while other areas will have a lower concentration of larger particles, with larger open spaces therebetween. Therefore, it is impossible to ensure that the various particles will achieve a uniform cutting pattern on the workpiece. The result is a relatively inefficient cutting tool.
The flat sided pyramid cutting insert is not particularly well suited to this cutting tool application, because each pyramid insert will almost certainly rest on one flat face, projecting a single point in the direction of rotation of the cutting tool. In this orientation, the three exposed flat side faces would be oriented at less than optimum angles for achieving the chip breaking effect.
Because of the relative inefficiency of the crushed tungsten carbide aggregate, the use of whole inserts arranged in a uniform pattern, with some type of chip breaking feature being employed, has come to be the industry standard for downhole milling and cutting. This efficiency has a price, however, in that the arrangement of cutting inserts in a uniform pattern, and the orientation of each insert at the optimum attack angle, add some expense and complexity to the cost of manufacturing the cutting tool. It is desirable to have a cutting element, and a method for applying cutting edges to a cutting tool, which will combine the simplicity of an aggregate cutting structure with the cutting efficiency of a uniform pattern of uniformly oriented identical cutting inserts.
The present invention can be summarized as a cutting element for use on a tool for cutting or milling metal items downhole, and a method of applying such cutting elements to such cutting tools. The cutting elements applied to a given cutting tool can be identically sized and shaped, and constructed of a uniform cutting grade material. Alternatively, a mixture of shapes can be employed, with each shape being designed to present an effective cutting contour to the workpiece. Each cutting element is composed completely of a plurality of faces, with each face having a basic geometric shape, such as an equilateral triangle, or a square. All of the faces of a given element can be identical. Throughout this application, the term xe2x80x9csubstantiallyxe2x80x9d is used. In general, the term xe2x80x9csubstantiallyxe2x80x9d should be understood to mean xe2x80x9cessentially or completely, with only insignificant exceptionsxe2x80x9d. More specifically, the term is used herein to describe a cutting element which is xe2x80x9csubstantiallyxe2x80x9d formed of a plurality of faces, with each such face having certain recited chip breaking characteristics. This means that all of the major faces are shaped to act as chip breakers. There could be very minor portions of the overall surface of the cutting element which are not thusly formed, but they are so minor that they do not alter the omnidirectional chip breaking function of the cutting element. Each face can be concave, in order to turn a metal chip back toward the workpiece surface and break it off at a short length. The cutting element can be cast of a high grade cutting formulation of tungsten carbide, or some other hard material. Alteratively, the cutting elements could conceivably be formed by other manufacturing processes. Each cutting element can have four, six, eight, or more concave faces. Each concave face of a cutting element can also have one or more surface irregularities therein, to act as additional chip breakers. These surface irregularities can be grooves, ridges, dimples, buttons, or other shapes capable of turning a metal chip back toward the surface of the workpiece.
Each cutting element is shaped so that, regardless of which face is bonded to the cutting tool, and regardless of the angular orientation of the cutting element, an effective cutting edge will always be applied to the workpiece. Each element is shaped so that it will have one of its faces bonded to the cutting tool, while the remainder of its faces are exposed. It does not matter which face is bonded to the cutting tool, because an arrangement of effective cutting faces will always be left exposed. Furthermore, this arrangement of effective cutting faces is designed so that, regardless of the angular orientation of the cutting element, an effective cutting edge will always be presented to the workpiece. Several shapes of cutting elements have been found to satisfy this requirement.
These cutting elements can be applied to the cutting tool in a substantially random fashion, such as the methods for application of the carbide particle aggregate discussed above, but the resulting pattern is far more uniform with the cutting elements of the present invention. This is because the cutting elements of the present invention are uniform in size and shape, so when tightly packed together, they tend to come to rest in a much more uniform pattern than would the varied assortment of crushed particles known in the prior art. Some of the cutting elements of the present invention are shaped such that several layers of the elements can be applied, in a relatively uniform fashion. Further, when the cutting elements of the present invention are applied to the cutting tool, the technician does not need to attempt to orient the individual cutting elements in any particular way. The cutting elements are designed so that, regardless of which face contacts the cutting tool, and regardless of how each cutting element is angularly oriented, an effective cutting edge will always be applied to the work piece.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: